Aliphatic polyamides, particularly nylon 6 and nylon 6,6 are extensively used for a variety of industrial and consumer applications such as carpets and automotive parts. Recycling and recovery of the polymer from these products is important for environmental reasons. The recovered polyamide materials need to be separated from foreign materials such as carpet backing, etc. While there are several approaches to reclamation through a chemical process, whereby the polymers or monomers can be purified and reused to make a high quality polyamide, these routes are expensive. A portion of the difficulties, inherent in the chemical process, relate to the complexity of the recyclable waste material.
Carpets include a face fiber that is adhered to a backing (support) material which may include jute, polypropylene, latex (such as a styrene-butadiene rubber (SBR)) and a variety of inorganic materials such as calcium carbonate, clay, or hydrated alumina fillers.
Nylon 6 and nylon 6,6 are often used for the face fiber in high quality carpets. Other face fibers includes polyethylene terephthalate, polypropylene, and to a much less extent wool and acrylics. Typically, a carpet comprises about 20-55 percent by weight face fiber and 45-80 percent by weight backing materials. In addition, the face fiber contains dyes, soil repellents, stabilizers, and other compounds added during fiber and/or carpet manufacture.
Waste carpet may also contain a host of other impurities, which will collectively be referred to herein as "dirt". A substantial amount of work is dedicated to processes where the polyamide (Nylon 6) is extracted from the waste and fed to a depolymerization process to recover caprolactam.
U.S. Pat. No. 5,241,066 to BASF describes a process for recovering caprolactam from waste containing both polycaprolactam and materials which are insoluble in acidic solvents for polycaprolactam. The process includes: agitating the waste in an acidic solvent capable of dissolving nylon 6 without degrading the acid insoluble materials; separating the solution containing nylon 6 from the insolubles; and feeding the nylon 6 to a depolymerization reactor to obtain pure caprolactam. Several deficiencies exist in this process. Industrial waste and post consumer carpets contain a mixture of polyamides (Nylon 6 and Nylon 66). The use of strong mineral acids, in this process, will extract both of these polyamides. At best only nylon 6 will be recovered to a monomer. These non-nylon 6 components interfere with caprolactam recovery. For example, one of the most difficult problems is that alkaline components, such as the calcium carbonate filler, neutralize the acidic catalysts, such as phosphoric acid, that are conventionally used to promote nylon 6 depolymerization, thus requiring the use of increased amounts of the catalyst. Extracted nylon 66 may interfere with the depolymerization of nylon 6 to caprolactam. Additionally, these types of processes, where only the monomer is recovered, loses the inherent value of the recovered polymer (i.e. one has to polymerize the monomer back to the polymer at some cost).
U.S. Pat. No. 5,310,905 teaches that a mixture of nylon 6 and nylon 66 is first separated from consumer waste, e.g. used carpet or carpet scrap, by extraction with an aliphatic carboxylic acid. The filtrate comprising the acid and extracted nylon 6 and nylon 66 is then depolymerized.
U.S. Pat. No. 5,241,066 teaches that a mixture of nylon 6 and polyethylene terephthalate, which is acid insoluble, is mixed with an acid so that the dissolved nylon 6 may be removed from the polyethylene terephthalate. The removed nylon is then depolymerized.
U.S. Pat. No. 3,317,519 teaches that a yarn blend of nylon 6 and polyethylene terephthalate may be depolymerized by heating with an aqueous alkali metal hydroxide at elevated pressure.
However, in the case of multi-component mixtures or composites that contain nylon 6 as one component, recovery of caprolactam is complicated by the presence of the other components. These other components and/or their decomposition products generated under conventional nylon 6 depolymerization conditions interfere with the isolation of caprolactam, thus necessitating expensive additional purification steps.
An alternative approach to the recovery of polyamides from waste is by the extraction of the polymers using a solvent, and the recovery of the polymer either by cooling the solution to cause precipitation of the polymer, or adding a non-solvent to precipitate the polymer, or removal of solvent by volatilization. The choice of a solvent system is critical to prevent degradation of the polymer, prevent extraction of undesirable materials, monitor the corrositivity of the solvent, control cost and monitor the environmental factors.
U.S. Pat. No. 5,430,068 describes a process for the extraction of polyamides from industrial and consumer application wastes. Boiling ethylene glycol is used to dissolve nylon 66 and the nylon was then allowed to remain in solution from 8 to 240 minutes, during which time the polymer degraded significantly according to the formic acid viscosity, where this value decreased substantially from the original value of 48.5 dl/g to 34.5 to 19.33 to 2.41, respectively. This rate of degradation is not acceptable because during an industrial extraction process, the polymer will be in contact with the solvent for at least 30 minutes.
Another extraction solvent described by this patent is glacial acetic acid. Although this solvent did not cause any change to the molecular weight, as judged by the lack of reduction of the solution viscosity, the solvent is reactive with the calcium carbonate present in the carpet backing causing dissolution of this inorganic material and foaming from the release of carbon dioxide. Moreover, the acid has to be substantially removed by extensive extraction with water to eliminate the acid whose presence in the polymer will cause degradation of the resultant recycled polymer during the subsequent melt processing steps.
Czechoslovakian Patent No. 143,502 claims recovery of caprolactam from waste by dissolving the polycaprolactam component in water or a polycaprolactam solvent under pressure and at high temperatures. The resulting low molecular weight polymer and oligomers are recovered by precipitation or used directly as a solution and then subjected to high temperature decomposition to recover the caprolactam. Two disadvantages exist in this teaching: the polymer recovered from this process can not be directly used since the process relies on breaking down the polymer for ease of dissolution and facile conversion to monomer. Another disadvantage of this teaching is that one can not use this method to recover waste containing both nylon 6 and nylon 66 since the extracted polyamide 66 will interfere with depolymerization to caprolactam.
German Patent No. DE 4421239, describes a process for recovering and processing high molecular weight nylon 6 from waste by depolymerizing the nylon 6 polymer in caprolactam and water mixtures, extracting the low viscosity oligomers, filtering the resultant solution and then repolymerizing to high molecular nylon 6 by subjecting the oligomeric materials to a vacuum at elevated temperatures in a reactor. An advantage of this method is that the resulting oligomer solution is suitable for filtration to remove non-soluble waste.
This method suffers from the lack of selectivity in the recovery of nylon 6, if nylon 66 is also present in the waste. The nylon 66 component, if present, will also be converted to oligomers and will be extracted along with the nylon 6 oligomers. Polymerization of this mix of polyamide oligomers will produce copolymers with a lower melt temperature and inferior mechanical properties than either nylon 6 or nylon 66.
European patent publication, 603,434 A1, describes a method for the separation of polyamides from polyamide containing products by dissolving the polyamides in a suitable, non-chlorinated solvent, filtration of the solution and precipitation of the polymer by adding non-solvent. Acid solvents suggested in this teaching, such as mineral acid and chloroacetic acid, will react with calcium carbonate causing foaming and extraction of this inorganic material into the polymer solution. Further complications arise during the recovery operation. The acid and inorganics presently retained in the polymer will cause instability and degradation of the recovered polyamide during application. Other solvents suggested in this patent such as phenol, cresol, benzyl alcohol, lactams or lactones will result in polymer solutions which have a high viscosity and are difficult to filter at any practical polymer concentrations. Selective separation of nylon 6 from waste containing nylon 66 is not addressed in this art.